In general, perovskite compounds refer to compounds having similar crystalline structure to that of calcium titanate mineral (perovskite). By molding and sintering such compounds, dielectric ceramics having dielectricity, piezoelectricity and semiconductive property can be obtained. Recently, such ceramics are widely used for capacitors, transmission filters, piezoelectric elements, thermistor, etc. for electric devices such as telecommunication devices and computer machines.
As described in Journal of Powder Technology, Vol. 34, No. 11, page 32, Kyoichi Sasaki, “Barium titanate, and process for production and process of the complex particles thereof” (1997), a solid phase process is well-known as a typical process for production of such perovskite compounds. For example, barium titanate, which is a typical example of perovskite compounds, is produced by a solid phase process in which barium carbonate and titanium oxide are heated to a temperature of 1000° C. or more to provide barium titanate. In such a solid phase process, the thus formed particles of barium titanate are aggregated each other so that the barium titanate is obtained as a fusion-bonded aggregate. Therefore, particles of barium titanate having a desired average particle size are conventionally obtained by crushing the aggregate by a mechanical crushing means, a media mill, or an airflow crusher.
As described in the Society of Powder Technology, Japan, ed. “Crushing, Sizing and Surface Modification”, page 99 (published in 2001), examples of the mechanical crushing means include, for example, a roll mill, a hammer mill, and a pin mill, among others, and examples of the media mill include, for example, a ball mill, a tube mill, a conical mill, a vibration mill, a tower mill, an atritor, a visco mill, a sand mill, and an annular mill, among others. Examples of the airflow crusher include, for example, a jet mill.
However, when a mechanical crushing means is used, the obtained powder is low in crushing degree. On the other hand, when a media mill is used, impurities are generated by abrasion of grinding media such as alumina, zirconia or agate jasper, or microparticles or chipping particles are generated by excess crushing and grinding of barium titanate by grinding media, and they contaminate the resulting powder of barium titanate. Therefore, the thus obtained powder of barium titanate has neither a satisfactory purity nor a satisfactory particle size distribution. Accordingly, a process using grinding media mainly formed of barium titanate has been suggested, as described in JP-A-03-174355 so that the resulting powder of barium titanate is not contaminated with impurities derived from grinding media, but it is still unavoidable that the resulting powder of barium titanate is contaminated with chipping particles.
As an alternative process for production of barium titanate powder, a wet process such as an alkoxide process, an organic acid salt process, a hydrothermal process, or a sol-gel process is also known. However, in the organic acid salt process and the sol-gel process, raw materials are reacted under heating to form an aggregate of barium titanate, and accordingly, it is necessary to crush the obtained aggregate of barium titanate, similarly to the solid phase process. Thus, also in these processes, it is unavoidable that the obtained powder of barium titanate is contaminated with impurities or chipping particles, as mentioned above.
On the other hand, as described in JP-A-05-330824, barium titanate can be directly synthesized by an alkoxide process and a hydrothermal process. However, these processes involve heat treatment to allow metamorphosis to a tetragonal system and to enlarge the particle size to some extent in view of practical need. Consequently, since also the thus formed barium titanate is obtained as an aggregate, these processes need a step for crushing the aggregate. Accordingly, also in the alkoxide process and the hydrothermal process, it is unavoidable that the obtained powder of barium titanate is contaminated with impurities or chipping particles.
As described above, according to the conventional process for production of powder of perovskite compound, the perovskite compound is obtained as an aggregate, and even when such an aggregate is crushed using a conventional mechanical crushing means, the aggregate is not crushed to a high degree. Then, when a media mill is used for crushing an aggregate to obtain a powder having a desired average particles size, it is unavoidable that the resulting powder is contaminated with impurities derived from chipping particles or grinding media. In turn, when an airflow crushing process is used, the particles of perovskite compound in the aggregate become distorted by impact of airflow during the treatment so that the obtained powder of perovskite compound does not necessarily have satisfying properties for obtaining homogeneous dielectric ceramics. Thus, there arises a problem, for example, that it cannot meet sufficiently to the demand for miniaturization and high performance of electronic parts such as capacitors, filters, thermistors, etc.
The invention has been accomplished to solve the above-mentioned problems involved in the production of powder of perovskite compound. It is an object of the invention to provide a process for production of powder of perovskite compound of which process provides a high purity powder of perovskite compound containing no impurities derived from grinding media or chipping particles, without resorting to impact caused by grinding media or airflow to crush an aggregate of perovskite compound.